The present invention relates generally to a method for enhancing the rust resistance and the surface finish of a non-ferrous workpiece, and more particularly to such a method useful in blastcleaning, profiling or shot peening non-ferrous materials, such as aluminum castings, using ferrous shot media.
Aluminum castings and other non-ferrous materials require surface preparation, cleaning, peening or profiling during the manufacturing process. These products are often blasted with conventional steel shot, such as tempered Martensite and/or a blend of Martensite steel shot and grit. The Martensite steel shot or grit fractures during use and deposits small fragments of ferrous materials into the surface of the aluminum or other non-ferrous castings. Further, steel grit generally etches the metal. Unless the casting undergoes some type of chemical or other post-treatment to remove the imbedded materials, this residue later forms rust on the surface of the non-ferrous parts upon exposure to air and moisture. However, there are drawbacks involved in chemical or other post-treatments, in that they are generally expensive, time consuming, require extra manufacturing facilities, and may damage the casting.
Aluminum oxide grit may also be used for aggressive cleaning, however, this etches the metal, leaves behind a residue, and has a relatively short useful life.
The conventional alternatives for blasting non-ferrous materials without leaving behind residue generally include the use of glass beads, sand, stainless steel shot, plastic chips, ceramic shot, salt, walnut shells, or the like. With the exception of stainless steel shot, for the other materials to be useful, the part must be exposed to the materials by blasting for excessive amounts of time; and the materials themselves generally have a relatively short useful life. As a result, these materials are generally too costly and are not feasible for a high production manufacturing facility. Stainless steel shot may be used in a conventional centrifugal wheel type blast machine, but the cost of the shot, as much as $7 per pound, is quite high as compared to Martensite shot or steel grit, either of which range between $.20 to $.30 per pound. Aluminum shot also may be used on aluminum, but this has a relatively short useful life, and is also quite expensive, as much as $3 per pound. As such, the cost of stainless steel or aluminum shot becomes prohibitive in most cases.
Thus, it is an object of the present invention to provide a method for enhancing the rust resistance and the surface finish of a non-ferrous workpiece, such as aluminum, brass or titanium alloy workpieces, using ferrous shot media or particles. It is a further object of the present invention to provide such shot media or particles which advantageously is available in great quantity and at low cost. Further, it is an object of the present invention to provide such shot media which does not fracture, thereby substantially avoiding imbedded particles, which may be prone to rust, in the workpieces. This may further advantageously eliminate any need for chemical or other post-treatment to remove rust-prone particulate matter. Still further, it is an object of the present invention to provide such shot media having a reduced hardness, without need for tempering, which may advantageously provide an enhanced surface finish as compared to conventional shot media. Yet still further, it is an object of the present invention to provide such shot media having a reduced hardness such that the blastcleaning, peening, or other machinery used may advantageously be comprised of less expensive, low grade steel wear parts.